Democratic congressional leaders vowed yesterday to use their powers of spending and policy oversight to challenge President Bush's expected proposal this week, as part of a broad revision of Iraq strategy, for boosting U.S. military forces in the country by as many as 20,000 troops.

Calling Iraq a nation in "complete chaos," House Speaker Nancy Pelosi (Calif.) and other Democrats cast the anticipated Bush plan as an escalation of the Iraq war that goes against the advice of senior U.S. commanders, rather than the significant change of course sought by American voters, and said that as a result they would treat the plan -- and new funding requests -- with strong skepticism.

"If the president wants to add to this mission, he's going to have to justify it," Pelosi said on CBS's "Face the Nation," emphasizing that while Congress will not cut off funding for troops now in Iraq, the White House will no longer have a "blank check" for expanding the war effort.

"When the bill comes . . . it will receive the harshest scrutiny," she said, referring to a new supplemental spending request for the wars in Iraq and Afghanistan that is expected to surpass $100 billion. Since Sept. 11, 2001, more than $500 billion has been spent on the wars and counterterrorism-related expenditures around the world, according to the Congressional Research Service.

The extra troops are only one part of the package Bush is expected to present to the nation as early as Wednesday. Proposals for economic development and the setting of political benchmarks for Baghdad, leading to the formation of a national reconciliation government, are also expected from Bush's prime-time speech, although final details were being ironed out over the weekend.

Democrats were divided Sunday over how much Congress can do on Iraq. Sen. Joseph R. Biden Jr. (Del.), who reaffirmed yesterday that he will run for the Democratic nomination for president in 2008, argued on NBC's "Meet the Press" that it would be unconstitutional for Congress to authorize the war but then cap troop levels or cut funding for specific items. Biden said any troop increase would be "a tragic mistake . . . but as a practical matter, there is no way to say, 'Mr. President, stop.' "

Republican lawmakers, for their part, voiced general support for the Bush plan as outlined, although they acknowledged it will be controversial and unpopular. Congress should not try to micromanage the war or cut off funding for U.S. troops, they said, warning that Democratic proposals for beginning a gradual U.S. military withdrawal in four to six months could lead to a "failed state" in Iraq and spell disaster for the Middle East.

"At the end of the day . . . I don't think Congress will cut off money for the troops," Senate Minority Leader Mitch McConnell (R-Ky.) said on "Fox News Sunday." "Congress is incapable of micromanaging the tactics in the war. And even though this will be a controversial step, I think the president will be able to carry it out, and I hope he'll be successful."

Sen. Lindsey O. Graham (R-S.C.), who along with Sen. John McCain (R-Ariz.) is a strong backer of more troops in Iraq, said the U.S. military from the beginning has lacked the resources it needs to "hold" areas cleared of insurgents. As a result, "if you're not winning, you're losing," he said on "Meet the Press." "We cannot let this country go into the abyss" of civil war, which could provoke intervention by Iran and Turkey and decades of regional conflict.

Graham said Bush's nomination last week of Lt. Gen. David H. Petraeus as the new U.S. commander for Iraq, replacing Gen. George W. Casey Jr., and his earlier naming of Robert M. Gates as defense secretary, were moves "long overdue."

Senior U.S. military officials have voiced deep resentment in recent days that the departures of Casey and Gen. John P. Abizaid, which had been planned for months, were being portrayed as part of a Bush administration housecleaning over Iraq strategy.

Some Republicans were more tepid on the subject of a troop increase. Brent Scowcroft, the former national security adviser to presidents Gerald R. Ford and George H.W. Bush and an early skeptic of the U.S.-led invasion of Iraq, offered only qualified support for a buildup. It would have to support a specific mission -- such as stemming ethnic cleansing in Baghdad -- for a specific period, he said.

"Unless there's something we can accomplish that is visible and demonstrable . . . people will say, well, look, the situation hasn't changed," he said in an interview on ABC's "This Week." "That demonstrates it's hopeless and the pressure to get out will increase."

Scowcroft also opposed making the U.S. commitment to Iraq dependent upon the Iraqi government accomplishing specific milestones -- an approach that is a central feature of Democratic withdrawal plans, and, according to U.S. officials, could also form part of the upcoming Bush plan.

"If they don't meet the first milestone, you withdraw support, making it virtually impossible for them to meet the second milestone," which would be "counterproductive," Scowcroft said. He stressed the necessity for a broader regional approach to bringing stability to Iraq, focusing on reviving Arab-Israeli peace efforts, but criticized the Bush administration's vision of bringing peace to the Middle East through democracy as "revolutionary utopianism."